For the convenience of the reader, applicant has added a number of topic headings to make the internal organization of this specification apparent and to facilitate location of certain discussions. These topic headings are merely convenient aids and not limitations on the text found within that particular topic.
In order to promote clarity in the description, common terminology for components is used. The use of a specific term for a component suitable for carrying out some purpose within the disclosed invention should be construed as including all technical equivalents which operate to achieve the same purpose, whether or not the internal operation of the named component and the alternative component use the same principles. The use of such specificity to provide clarity should not be misconstrued as limiting the scope of the disclosure to the named component unless the limitation is made explicit in the description or the claims that follow.
This invention relates to 2D and 3D ultrasound phased array imaging systems for transmitting and receiving ultrasound energy. In particular, this invention relates to an improved sparse array structure that provides an effective aperture and radiation pattern comparable to that of a dense array having a far greater number of array elements. The preferred embodiment of the present invention uses separate arrays of transmit and receive elements and does not employ dual-use xe2x80x9cshared transmit/receive elementsxe2x80x9d. The use of two sets of elements allows for a very simple receive buffer to be placed in the head of the transducer and allows the transducer to utilize different transducer optimization between transmit and receive elements. One such case would be the use of two distinct frequencies for harmonic processing and other advantages.
This invention adds to the body of work of image processing. Much of the interest and the earlier work has been in the field of medical image processing. FIG. 1 introduces the components of a medical imaging device 100. The medical imaging device comprises a main body 104 connected to a display 108 and various input devices such as a keyboard 112. A cable 116 containing a set of wires 120 connects an array 124 of transducer elements 128 in the instrument head 132 to the main body 104.
In medical imaging, the array of transmit elements 128 sends coordinated pulses of energy into the body. The energy bounces back in various directions as the transmitted energy hits the various surfaces of items within the body. The goal is to measure the energy that reflects back and use the measurements to deduce information about the tissue within the body. Most people know that the thunder from a single clap of lightning is heard by people in a village at different times depending on the relative position of the various observers to the lighting. The same principle applies in imaging in that there is information in not just the amount of energy received by the receive elements but in the delay between the transmission of the energy and the receipt of the echo.
In order to get useful information, there are many transmission elements and many receive elements. Timing delays are used so that elements at different distances from a particular piece of the target can be used collectively to form an image of that piece of the target.
This invention builds on prior concepts and addresses the never-ending quest to get more for less. In this case, xe2x80x9cmorexe2x80x9d means higher resolution images with less artifacts. In this context, xe2x80x9clessxe2x80x9d means using fewer resources such as fewer devices to transmit or receive the measurement signals, and fewer resources to coordinate, control the devices, and process the information acquired from the receive devices.
The goal is to get the information content that is close to as good as would be obtained from a large fully-populated array of shared transmit/receive elements set out in n rows and n columns while using much less than nxc3x97n elements. Such an array with fewer than nxc3x97n elements is called a sparse array.
One improvement in the search for getting more for less is described in U.S. Pat. No. 5,537,367 to Lockwood et al. for Sparse Array Structures. The Background of the Lockwood patent sets forth the problem.
Arrays of transducers are used to transmit and/or receive electromagnetic or acoustic energy over a specified region of space (the target).
A portion of the energy that is transmitted bounces back each time the energy wave reaches a new surface in the scanned material.
The arrays of elements are controlled with phase shifts (timing delays) and possibly by weighting so that signals sent to or received from the target constructively interfere while signals outside of the target destructively interfere.
The radiation pattern is a plot of the amplitude of the signal transmitted or received by the array as a function of the position in space.
The radiation pattern of an array indicates how well the array achieves the desired constructive and destructive interference. The radiation pattern is usually plotted in polar coordinates at a given distance in front of the array.
The transmit/receive radiation pattern is a combination of the transmit radiation pattern and the receive radiation pattern. The transmit/receive radiation pattern gives a measure of the sensitivity and resolution with which the array will be able to detect objects.
An example of a typical transmit-receive radiation pattern is shown in FIG. 2. The radiation pattern consists of a prominent main lobe 150 and a number of secondary lobes 154. The main lobe corresponds to the desired region in space over which energy will be transmitted and from which energy will be received.
The width of the main lobe 150 is inversely proportional to the width of the array and determines the resolution of the array. In other words, a larger array has a narrower main lobe 150 and has better resolution. Lobe width is often described as the distance between the lobe""s xe2x80x9cshouldersxe2x80x9d.
The secondary lobes 154 are caused by imperfect destructive interference outside of the target area and result in the transmission and reception of unwanted energy. The energy received from side lobes does not represent information about the target region and makes it more difficult to detect subtle differences in the target.
Thus, it is a goal when designing an array to minimize the width of the main lobe (to increase resolution) while minimizing the secondary lobes.
One way to get a higher quality image is to add transducer elements to make a larger fully-populated array. Adding array elements has benefits. Experience has shown that the array should be at least as wide as 20 times the wavelength of sound at the transducers"" center frequency in order to focus sharply to give high imaging resolution.
However, there are practical limits on how many array elements can be added. The costs associated with adding array elements include:
1) the cost of the additional transmit or receive element,
2) the cost of the circuits to control or process the information from the element,
3) the problem of cross coupling that occurs when too many wires connected to the various transmit and receive elements are running through the cable connecting the head to the main body of the measurement instrument,
4) the undesired weight added to the measurement instrument head; and
5) if transducers are shared for both transmitting and receiving, it will require transmit/receive switching with its added costs and size.
A) Increase the Spacing Between Elements
One solution to the desire to have a wide array but control the number of elements would be to use a fully populated array but simply space the elements with greater gaps between them. However, whenever the spacing of elements in a phase array transducer exceeds approximately one half of the wavelength of sound used by the element, the periodic spacing of the elements causes additional unwanted side lobes known in the art as grating lobes.
Since it is desirable that the array should be at least 20 wavelengths (20 lambda) wide in order to provide high imaging resolution and it is desirable to have the elements spaced at no more than 0.5 lambda, a fully populated 20 lambda array would be an expensive proposition. Such a system would need to be capable of pulsing 1600 transmit elements and operate 1600 receive elements.
B) Vary the Spacing to Reduce Grating Lobes
The grating lobes can be reduced by having variations in the spacing between elements. It is difficult to design such aperiodic arrays in a way that minimizes the secondary lobes. Attempts to use random connection patterns broke up the strong grating lobes into numerous small grating lobes. This avoided strong ghost images but led to a low contrast image as the many small grating lobes picked up echoes from many different directions and produced a visible background clutter.
Others have tried a vernier approach with different uniform patterns for overlapping transmit and receive arrays. For example, one might try to use every other element for transmit and every third element for receive. There were attempts to get the receive grating lobes to be non-aligned with the transmit grating lobes. One scheme was to design a vernier array with the transmit grating lobes falling at angles corresponding to the nulls in the receive beam pattern. While the vernier approach was an improvement over the random pattern, the result still had ghost images.
C) Apodization
The solution chosen in the Lockwood patent is a process known as apodization. Apodization is the process of weighting the individual array elements in an attempt to optimize the array performance. Apodization is not without costs. Apodization causes a wider main beam and reduces the signal to noise ratio. The reduced signal to noise ratio is a consequence of attenuating the signals that are already small before attenuation.
An example of a sparse array designed for use with apodization is included as FIG. 3. This figure shows an array disclosed in the Lockwood ""367 patent.
This two dimensional array has 69 receiver elements and 193 transmit elements. The array was designed with two overlapping arrays, one for transmit and one for receive. The points where transmit and receive arrays overlap use combination shared transmit/receive elements and the associated circuits to enable the sharing of the transmit/receive capabilities of the element.
For example, a shared transmit/receive element shares one wire in the cable that goes from the head to the main body of the measurement instrument. Multiplexer switches must switch the connections from the high voltage pre/amp needed for the transmit portion of the element and low voltage pre/amp needed to operate the receive portion of the element.
In accordance with the Lockwood method, the elements were weighted with an apodization function (in this particular case, it was a cosine apodization).
D) Disadvantages of the Method Proposed by Lockwood
The method taught by the Lockwood patent suffers from several drawbacks.
The first drawback is the previously described complication associated with having to use an apodization scheme to weigh the individual transmit and/or the individual receive elements in order to obtain a suitable radiation pattern.
In addition to the drawbacks from the use of apodization, there are drawbacks from the use of the shared transmit/receive elements that are necessitated by the overlap between the receive and transmit arrays. The drawbacks from the use of the shared use elements go beyond the need for multiplexing switches and the need for coordinating the high speed switching of the circuits and wire from one function to the other.
One drawback is the need to protect the ultra low voltage receive circuitry from the high voltages used by the transmit circuit. This problem does not exist with a sparse array using dedicated transmit elements that are separate from the dedicated receive elements. A transmit element that does not share circuitry with a receive element can operate at a higher voltage (on order of magnitude of tens or hundreds of volts) without risk of damaging the sensitive receive circuitry which operate on voltages from 2 to 8 orders of magnitude less than the transmit circuits (on the order of micro-volts). Thus, there is less need for circuitry to isolate the high voltage transmit circuits from the low voltage receive circuits.
Another drawback is that the shared transmit/receive element operates on a single frequency range and there may be reasons why it would be advantageous for the transmit and receive elements to operate on different frequency ranges. The prior art has found advantages in certain imaging techniques that use one frequency for the transmitted energy and measure the received energy at another frequency. This is called harmonic imaging. The receive frequency is often a multiple of the transmit frequency but it could be a sub-harmonic (such as xc2xd of the transmit frequency) or a fractional harmonic (such as 3/2 the transmit frequency). When using a shared transmit/receive element intended to transmit a first frequency and receive back a different frequency, one has difficulty optimizing both the transmit and the receive functions as there is only one transducer crystal with a single range of transducer frequencies.
E) Annular Arrays and Steerable Limited Diffraction Beams
One way of characterizing two-dimensional ultrasonic arrays is to divide arrays into linear arrays and annular arrays. Linear arrays have elements arranged in a line or a grid pattern. These individual elements can be activated to form regions of active elements in shapes such as the circles, rings, or polygons.
In contrast, annular arrays have a series of ring shaped transducers that are arranged in a concentric pattern. The center of the annular array can be a circular transducer. Instead of having hundreds of separately controlled elements, an annular array may have simply a center element and three annular elements surrounding the center. The annular arrays thus have fewer elements and this would seem to be an advantage. However, it is actually a disadvantage as having fewer elements provides less opportunity for control through time delays for pulsing and measurement windows, weighting, and other processing tools.
As mentioned, one noted shortcoming with annular arrays is that the beams cannot be electronically steered by introduction of time delays as is commonly done in linear arrays. Thus, steering must come from mechanically scanning the head or from a mechanical wobble that moves the head. Mechanical movement has a number of shortcomings relative to electronic steering, such as limitations from inertia, problems with wear, and the inability to collect data in a pattern that is independent of the mechanical properties of the mechanical equipment.
The prior art has worked with annular arrays to obtain approximate a Bessel beam with a large depth of field and depth-independent properties.
One recent paper by J. Lu and J. F. Greenleaf titled xe2x80x9cA Study of Two-Dimensional Array Transducers for Limited Diffraction Beamsxe2x80x9d appeared in the IEEE Transactions of Ultrasonic, Ferroelectrics and Frequency Control at vol. 41, U.S. Pat. No. 5,724,739, September 1994 described the approximation of an annular array by use of elements in a two-dimensional linear array so that electronic beam steering could be used to steer the annular regions of elements.
This paper suggested the use of 14 rings excited according to a Bessel function weighting pattern. The goal would be to retain the favorable focal qualities of a limited diffraction beam while adding the beam steering capabilities of a two-dimensional array.
Instead of using annular (single element) rings, each ring would be comprised of a set of transducer elements used for both the transmit and the receive functions. The entire set of transducers for a ring would be electronically connected so that all the elements in a particular ring would be driven by the same waveform. The paper suggests the use of a series of elliptical shaped rings, instead of circular rings, in order to compensate for the reduction in effective aperture from steering the beam. As the beam is steered further from the center, the device switches to progressively larger elliptical shaped-rings to offset the decrease in effective aperture from the increase in steering angle.
The advantage of this approach is an extended transmit focal zone that is characteristic of Bessel beams or other limited diffraction beams.
One limitation of this approach is the use of weighting or apodization because of the cost of the added circuit complexity. Another, limitation is the large number of elements required for this scheme. Even spaced at the relatively large interelement spacing of 1.5 lambda, the elliptical array of 29.2 lambda by 41.2 lambda requires 1700 shared transmit/receive elements.
Even with the extensive work in the prior art, there remains a need for a two-dimensional sparse array that provides a high quality image from a relatively small total number of transmit and receive elements.
It is an object of the present invention to provide a two-dimensional sparse array that provides a high quality image from a relatively small number of transmit and receive elements and overcomes limitations found in prior art solutions.
These and other advantages of the present invention are apparent from the drawings and the detailed description that follows.
This disclosure a novel sparse array that uses a small fraction of a fully populated array but yields a radiation pattern that is suitable for high quality medical imaging.
More specifically, the disclosed sparse array uses an inner array of transmit elements and an outer array of receive elements. In the preferred embodiment, the system using this sparse array does not use apodization for weighting. In the preferred embodiment, the system does not use shared transmit/receive elements and thus avoids the costs and compromises inherent in using shared transmit/receive elements.
In the preferred embodiment, the elements are spaced at no more than approximately 0.5 lambda of a center frequency for the elements. This inter-element spacing applies to both the scan (azimuth) and elevation directions.
The preferred embodiment discloses arrays with a transmit aperture that is a narrow aperture (broad beam) to xe2x80x9cilluminatexe2x80x9d the target area with a receive aperture that is a large aperture (narrow focus) which is well suited to use in applications using parallel processing.